During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Traditionally, press sections have included a series of nips formed by pairs of adjacent cylindrical press rolls. In recent years, the use of long nip presses has been found to be advantageous over the use of nips formed by pairs of adjacent press rolls. This is because the longer the time a cellulosic fibrous web can be subjected to pressure in the nip, the more water can be removed there, and, consequently, the less water will remain behind in the web for removal through evaporation in the dryer section.
The present invention relates to long nip presses of the shoe type. In this variety of long nip press, the nip is formed between a cylindrical press roll and an arcuate pressure shoe. The latter has a cylindrically concave surface having a radius of curvature close to that of the cylindrical press roll. When the roll and shoe are brought into close physical proximity to one another, a nip, which can be five to ten times longer in the machine direction than one formed between two press rolls, is formed. This increases the so-called dwell time of the cellulosic fibrous web in the long nip while maintaining an adequate level of pressure per square inch of pressing force. The result of this long nip technology has been a dramatic increase in dewatering of the cellulosic fibrous web in the long nip when compared to conventional press nips on paper machines.
A long nip press of the shoe type requires a special belt, such as that shown in commonly assigned U.S. Pat. No. 5,238,537 to Dutt. This belt is designed to protect the press fabric supporting, carrying and dewatering the cellulosic fibrous web from the accelerated wear that would result from direct, sliding contact over the stationary pressure shoe. Such a belt must be provided with a smooth, impervious surface that rides, or slides, over the stationary shoe on a lubricating film of oil. The belt moves through the nip at roughly the same speed as the press fabric, thereby subjecting the press fabric to minimal amounts of rubbing against the surface of the belt.
Traditional methods of making a long nip press belt involve the use of yarns and liquid resin systems. In particular, there are three known methods for fabricating a belt using these materials. The first method uses a two-roll system in which an endless woven substrate is coated with a liquid urethane resin. The second method employs a building mandrel upon which reinforcing yarns are laid up on the outside surface of the mandrel in an array essentially 90 degrees to each and then totally encapsulated with liquid urethane resin. The third method is similar to the second with the difference being that the inside surface of a mandrel is used to lay the strands and pour the resin to form the belt.
With all three methods, the yarns used to reinforce the structure are either monofilament or multifilaments and are positioned in the resin to protect their integrity. Due to the relatively large size of the reinforcing yarns and the amount of resin material required to encapsulate the yarns, the caliper of the belt can become prohibitively thick. This is especially so in the case of grooved or blind drilled belts which require additional resin caliper into which the grooves or holes are machined.
The present invention solves this problem by forming a belt using pre-impregnated tape. The tape comprises individual filaments laid side by side in a ribbon like fashion, and encapsulated and protected with thermoplastic resin (see FIG. 1) The use of thermoplastic-impregnated filaments enables rein-forcing elements to be put into a belt structure without substantially increasing the belt caliper. These individual filaments are smaller than yarns that are comprised of bundles of filament, as used in the manufacture of conventional belts. This “prepreg” tape is the building block of the present invention.